In the treatment of fuel gases such as coke oven gases and the like which contain ammonia, hydrogen cyanide and hydrogen sulfide a common treating method is to contact the gas stream with an ammonium polysulfide wash. In this technique the ammonium polysulfide reacts with the hydrogen cyanide and ammonia to form ammonium thiocyanate, the reaction being represented by the following equation: EQU (NH.sub.4).sub.2 S.sub.(x.sub.+1) + HCN + NH.sub.3 .fwdarw. NH.sub.4 SCN + (NH.sub.4).sub.2 S.sub.x (1)
In the practice of this method, finely divided sulfur is added to the solution in order to maintain the sulfur concentration required to convert all or nearly all of the hydrogen cyanide to ammonium thiocyanate.
The process is deficient in that it requires the constant addition of sulfur to the solution representing a raw material expense. In addition to the difficulties presented in dissolving the sulfur, problems exist with respect to disposal of the ammonium thiocyanate which is toxic in nature without causing pollution problems.
It has been proposed by others to incinerate the waste liquor to form a sulfurous vapor which is hydrogenated to form hydrogen sulfide. The hydrogen sulfide so formed is removed from the gas stream by conventional processes such as the Stretford Process in which the hydrogen sulfide is converted to sulfur.
If sulfur is added to the system in order to remove the hydrogen cyanide and the sulfur subsequently converted to hydrogen sulfide which is, in turn, reconverted into sulfur, it introduces a sulfur recycle stream to the process which increases the load on the hydrogen sulfide removal processing steps.
For example, in the treatment of 45 million standard cubic feet of coke oven gas typically containing about 15,125 lbs. of sulfur as hydrogen sulfide and 3,857 lbs. of hydrogen cyanide, it is required that at least 4,571 lbs. of sulfur be present to react to the hydrogen cyanide to form ammonium thiocyanate.
When the ammonium thiocyanate is subsequently converted to hydrogen sulfide and ammonia, it represents an increase in the hydrogen sulfide content of the gas stream by a factor of at least 1.3. This represents the ideal case which assumes 100% utilization of the sulfur in the solution prior to the subsequent reduction of hydrogen sulfide which is in turn converted to sulfur which must in part be recycled. More typically, the excess of sulfur is present in the solution for effective reaction of hydrogen cyanide and the factor is usually about 1.5 times or more.
As a consequence, this necessitates the use of a correspondingly larger and more costly hydrogen sulfide removal plant and increases the operating cost of this plant in proportion to the increased amount of hydrogen sulfide throughout.